1. Field of the Invention
The present invention relates to a method of warming up a reheat turbine, and more particularly to a method of warming up a high pressure steam turbine in a reheat turbine which is equipped with a high pressure steam turbine and an intermediate pressure steam turbine and employs an intermediate pressure steam turbine starting system.
2. Description of the Prior Art
In conventional reheat turbines employing an intermediate pressure steam turbine starting system, an intermediate pressure steam turbine is started under the state where the interior of a high pressure steam turbine is kept in vacuum. After a predetermined transfer load is attained (that is, after an inlet steam flow regulating valve of the intermediate pressure steam turbine is fully opened), a rated load is reached by opening gradually a steam flow regulating valve of the high pressure steam turbine.
The inflow steam quantity to the high pressure steam turbine is limited in order to prevent a drastic increase of a rotor thermal stress resulting from the difference (a mismatch quantity) between a low temperature steam turbine metal temperature rotating in vacuum and a high pressure steam temperature that flows into the high pressure steam turbine.
The mismatch quantity will be explained with reference to FIG. 4. At the start of the intermediate pressure steam turbine, a mismatch quantity t.sub.1 is generated between the metal temperature T.sub.m1 of the reheat steam chamber and the temperature T.sub.s1 of the inflow steam in the reheat steam chamber as shown in the curves A and B, respectively. A necessary time till the ventilation of the high pressure steam turbine is determined so that the thermal stress generated thereby is below an allowable value.
As described already, it is necessary for the metal temperature to follow rapidly the steam temperature with a greater mismatch quantity, and at the same time, the speed as well as the load must be raised in the course of a long period. This exactly holds true of the start of the high pressure steam turbine.
The steam temperature at the high pressure first stage of the inflow steam at the time of the start of the high pressure steam turbine is hereby called T.sub.s2 as shown in curve C. In accordance with the conventional technique, the metal temperature T.sub.m2 ' at the high pressure first stage as shown in curve E is the same temperature as the temperature before the start of the intermediate pressure steam turbine because no warm-up is made at all. Therefore, the mismatch quantity at the high pressure first stage at the time of ventilation of the high pressure steam turbine is .DELTA.t.sub.2 ' as shown in FIG. 4.
The above method of warming up the reheat turbine employing the intermediate pressure steam turbine starting steam by utilizing the intercept bypass valve relatively speeds up the timing of opening the steam flow regulating valve at the inlet of the high pressure steam turbine and shortens the start time.
However this conventional method does not take into consideration any measures for improving the mismatch quantity described above, and hence is not free from the problem that the high pressure steam turbine cannot be sufficiently protected from the adverse influence of the thermal stress.
On the other hand, for example, in Japanese Patent Publication No. 2483/1985, a conventional method of warming up a reheat turbine employing an intermediate pressure steam starting system is proposed.
According to above conventional method of warming up a reheat turbine, an intermediate pressure steam turbine provides an intercept bypass valve for regulating flow steam having a small diameter thereon in addition to an inlet steam flow regulating valve. The intercept bypass valve for regulating flow steam is opened before the start of the intermediate pressure steam turbine, and after a predetermined tranfer load is attained, an another steam flow regulating valve at a high pressure steam turbine inlet is opened and the high pressure steam turbine is then started.